// *=> string null和undefined无法使用
var num = 100
num = num.toString()
console.log(num, typeof num);

// 数组
var arr = [1, 2, 3, 4, 5, 6]
arr = arr.toString()
console.log(arr, typeof arr);

var obj = { name: 'zhangsan' }
obj = obj.toString()
console.log(obj, typeof obj);

// *=> boolean
console.log(Boolean('hello'));

// *=> number 
console.log(Number('100'));